Magnesium diboride (MgB2) has a critical temperature (transition temperature) of 39 K, which is higher than the critical temperatures of conventional superconductors (for example, niobium-titanium (NbTi) and triniobium-tin (Nb3Sn) and the like). Furthermore, unlike a wire using an oxide superconductor, a wire using MgB2 has an advantage that, when a closed circuit using the wire is operated at a persistent current mode, the wire has high electrical field stability.
The persistent current mode is an operation method in which a current is continuously flown in a closed circuit formed by using a superconductor. That is, since a superconducting wire has a resistance of zero, once a current is flown in a closed circuit, the current is continuously flown without attenuation. In order to attain such persistent current mode, a technique to joint end parts of superconducting coils or superconducting wires constituting a persistent current switch with a superconductor is important. In addition, superconducting wires are generally used as multi-core wires constituted by a plurality of filaments in view of current capacity, wire length, magnetic stability and alternate current loss, and thus are demanded to be capable of joint multi-core wires.
PTL 1 describes a method including polishing tip ends of wires containing a mixed powder of magnesium (Mg) and boron (B) or MgB2 wires to expose MgB2 cores, inserting the wires in a container, filling the container with a mixed powder of Mg and B from the direction orthogonal to the wires, pressurizing the mixed powder, and conducting a heat treatment. By the heat treatment, a sintered body of MgB2 is formed, and the wires are jointed.